1. Field of the Invention
The present invention relates to a thin-film circuit substrate in which an organic insulating film formed on the surface of the substrate and a metallic wiring (electrode material) formed on the insulating film have high adhesion to each other, a method of producing the same, and a high frequency module using the thin-film circuit substrate.
2. Description of the Related Art
In the recent radio communication field, high frequency devices are required to have small sizes, low costs, and high performances.
Moreover, the high frequency devices need to be provided with transmission lines having a low transmission loss and a high efficiency. Wiring in the devices is carried out by using an electrode material (metallic material) having a low resistance. Moreover, an inter-layer insulating film between wires needs to be formed with a dielectric material having a low relative dielectric constant and a low dielectric loss tangent.
Thus, to satisfy the above-described requirements, various types of circuit substrates have been developed in which low resistance materials such as Au, Cu, Ag, Al, and so forth are used as the electrode material, and organic resins having a low dielectric constant and a low dielectric loss tangent, such as polyimide, epoxy resins, benzocyclobutene resins, bismaleimide triazine resins, and so forth are used as the dielectric material.
However, in thin-film circuit substrates containing a combination of organic resins and an electrode material, bonding strengths between the organic resins and the electrode material are insufficient. This causes a problem in that film-peeling occurs during processes of forming a metal wiring, bonding wires, and so forth. Thus, to increase the bonding strength between an organic resins and an electrode material, for example, the following various methods have been proposed, in which
(1) an adhesion layer made of a metal having a relatively high bonding strength to an organic resin is formed as an adhesion layer between an electrode material and the organic resin,
(2) the surface of an organic resin is oxygen-plasma treated to enhance the bonding strength thereof (Japanese Unexamined Patent Application Publication No. 8-134639).
(3) a polar polymer is formed on an organic resin to improve the adhesion between the organic resin and the polar polymer (Japanese Unexamined Patent Application Publication No. 9-219586), and
(4) the surface of an organic insulating film is cleaned, activated, and surface-roughened by plasma treatment.
Referring to the above-described method (1) in which the metal film having a high adhesion strength to the organic resin is formed as the adhesion layer between the organic resin and the electrode material, different types of metals such as Cr, Ti, Ni, Nb, V, and so forth are used for the adhesion layer. Film-peeling during a wiring process can be prevented, due to the operation and effects of the adhesion layer made of such a metal as mentioned above. However, the method (1) is ineffective in preventing film-peeling during a wire-bonding process in which supersonic waves and high temperature loads are applied. In fact, this method (1) is not a satisfactory countermeasure against film-peeling.
Referring to the method (2) in which the surface of the organic resin is surface-treated by oxygen-plasma, the adhesion strength between the electrode material and the organic resin can be enhanced. However, the surface of the organic insulating film is oxidized. This causes a problem in that the electrical characteristics such as the relative dielectric constant and the dielectric loss tangent are degraded so that target characteristics for the high frequency module can not be obtained.
Referring to the above-described method (3) in which the polar polymer is formed on the organic resin to enhance adhesion between the organic resin and the metal, a process of polymerizing polar monomers after the surface of the organic resin is activated is required. Although the adhesion between the metal and the organic resin is enhanced, the time required for the processing is long. Thus, a problem arises in that the manufacturing cost of the high frequency module increases.
Referring to the method (4) in which the surface of the organic insulating film is cleaned, activated, and surface-roughened by plasma treatment to enhance the adhesive properties of the organic insulating film, the treatment is carried out in the atmosphere, which causes the surface of the organic insulating film to be oxidized. Thus, a problem arises in that the electrical characteristics of the organic resin such as the relative dielectric constant, the dielectric loss tangent, and so forth are deteriorated. Moreover, the surface-roughness at the surface of the organic insulating film is large, and the rough surface-features are transferred to the wiring formed on the organic insulating film. Thus, problematically, the resistance is increased, so that target characteristics for the high frequency module can not be obtained in millimeter wave or microwave regions.
Accordingly, it is an object of the present invention to provide a thin-film circuit substrate in which an organic insulating film on the surface of a substrate and a metal wiring (electrode material) formed thereon have high adhesive properties for strongly adhering to each other, a method of producing the same, and a high frequency module using the thin-film circuit substrate.
To achieve the above-described object, according to the present invention, there is provided a thin-film circuit substrate which comprises a substrate, an organic insulating film formed on the surface of the substrate, and a metal wiring made of a thin-film metal formed on the organic insulating film, wherein the surface of the organic insulating film is provided with a surface modification layer having a surface modification coefficient (SMC) defined by the following formula:                     SMC        =                              the            ⁢                          xe2x80x83                        ⁢            total            ⁢                          xe2x80x83                        ⁢            number            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            functional            ⁢                          xe2x80x83                        ⁢            groups                                                                                                    the                    ⁢                                          xe2x80x83                                        ⁢                    total                    ⁢                                          xe2x80x83                                        ⁢                    number                    ⁢                                          xe2x80x83                                        ⁢                    of                    ⁢                                          xe2x80x83                                        ⁢                    C                    ⁢                                          xe2x80x83                                        ⁢                    atoms                    ⁢                                          xe2x80x83                                        ⁢                    present                    ⁢                                          xe2x80x83                                        ⁢                    at                    ⁢                                          xe2x80x83                                        ⁢                    the                                    ⁢                                      xe2x80x83                                                                                                                        surface                  ⁢                                      xe2x80x83                                    ⁢                  of                  ⁢                                      xe2x80x83                                    ⁢                  the                  ⁢                                      xe2x80x83                                    ⁢                  organic                  ⁢                                      xe2x80x83                                    ⁢                  insulating                  ⁢                                      xe2x80x83                                    ⁢                  film                                                                                        (        1        )            
wherein the surface modification coefficient is between 0.1 and 0.5.
In the thin-film circuit substrate in accordance with the present invention, the surface modification layer having a surface modification coefficient of 0.1 to 0.5 is provided at the surface of the organic insulating film formed on the substrate, and the metal wiring is formed on the surface of the organic insulating film having the surface modification layer. Accordingly, the bonding strength between the metal wiring (electrode material) and the organic insulating film can be enhanced without deteriorating the electrical characteristics such as the relative dielectric constant and the dielectric loss tangent.
In the present invention, the term xe2x80x9csurface modification coefficientxe2x80x9d is defined as the ratio of the number of C atoms constituting functional groups to the number of C atoms present at the surface of the organic insulating film. The surface modification coefficient (SMC) is expressed by the following formula:                     SMC        =                              the            ⁢                          xe2x80x83                        ⁢            total            ⁢                          xe2x80x83                        ⁢            number            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            functional            ⁢                          xe2x80x83                        ⁢            groups                                                                                                    the                    ⁢                                          xe2x80x83                                        ⁢                    total                    ⁢                                          xe2x80x83                                        ⁢                    number                    ⁢                                          xe2x80x83                                        ⁢                    of                    ⁢                                          xe2x80x83                                        ⁢                    C                    ⁢                                          xe2x80x83                                        ⁢                    atoms                    ⁢                                          xe2x80x83                                        ⁢                    present                    ⁢                                          xe2x80x83                                        ⁢                    at                    ⁢                                          xe2x80x83                                        ⁢                    the                                    ⁢                                      xe2x80x83                                                                                                                        surface                  ⁢                                      xe2x80x83                                    ⁢                  of                  ⁢                                      xe2x80x83                                    ⁢                  the                  ⁢                                      xe2x80x83                                    ⁢                  organic                  ⁢                                      xe2x80x83                                    ⁢                  insulating                  ⁢                                      xe2x80x83                                    ⁢                  film                                                                                        (        1        )            
Hereinafter, a method of determining the surface modification coefficient, employed in the present invention, will be described in more detail with reference to FIG. 5.
To determine the surface modification coefficient, the ratios of C atoms contained in the respective bonds at the surface of an organic insulating film are identified by X-ray photoelectron spectroscopy.
For example, FIG. 5 shows the C1s spectra of the surface of the organic insulating film (benzocyclobutene resin) obtained by the X-ray photoelectron spectroscopy. The ratios of the areas of the spectra in FIG. 5 represent the ratios of the number of C atoms contained in the respective bonds. Moreover, the sum of the areas of the spectra represents the total number of C atoms present in the detected area. In particular, the percentage of C atoms contained in the Cxe2x80x94H or Cxe2x80x94C bonds, the xe2x80x94Cxe2x95x90O bonds, and the xe2x80x94COO bonds detected as shown in FIG. 5 are 84%, 10%, and 6%, respectively. The percentage of C atoms constituting the functional groups at the surface of the organic insulating film is 16%, which is the sum of 10% for the xe2x80x94Cxe2x95x90O bonds and 6% for the xe2x80x94COO bonds. In this case, the surface modification coefficient, calculated by the formula (1) is 0.16 ({fraction (16/100)}=0.16).
In the present invention, the term xe2x80x9cmetal wiringxe2x80x9d includes an electrode, a pad, and so forth, in addition to a transmission line.
The shape and size of the substrate and materials constituting the substrate have no special limitations. Substrates made of different materials and formed in different shapes and sizes may be employed.
The metal wiring may have a single-layered structure, or may have a multi-layered structure, namely, an at least two layer structure which includes a metallic material layer as an adhesion layer and a layer formed on the adhesion layer, made of a metallic material different from that for the adhesion layer.
According to the present invention, the surface modification coefficient is in the range of 0.1 to 0.5. If the surface modification coefficient is less than 0.1, the bonding strength between the metal wiring and the organic insulating film will be insufficient. Moreover, if the surface modification coefficient is more than 0.5, the electrical characteristics of the organic insulating film may be deteriorated.
Preferably, at least one functional group selected from the group consisting of a sulfonic group, a carboxyl group, a carbonyl group, and a phenol group is present at the surface of the surface-modification layer.
By providing the thin-film circuit substrate with the organic insulating film in which at least one functional group selected from the group consisting of a sulfonic group, a carboxyl group, a carbonyl group, and a phenol group is present at the surface of the surface-modification layer, the bonding strength between the metal wiring (electrode material) and the organic insulating film can be enhanced with the electrical characteristics such as the relative dielectric constant and the dielectric loss tangent being maintained at a satisfactory level.
Preferably, the metal wiring on the organic insulating film is made of a material containing at least one metal selected from the group consisting of Cu, Au, Ag, Al, Ni, Ti, Cr, NiCr, Nb, and V.
By using the metal wiring made of a material containing at least one metal selected from the group consisting of Cu, Au, Ag, Al, Ni, Ti, Cr, NiCr, Nb, and V, a thin-film circuit substrate with a high reliability, provided with the metal wiring having a low resistance and a high bonding strength for the organic insulating film, can be obtained.
Preferably, the organic insulating film contains at least one resin selected from the group consisting of polyimide resins, epoxy resins, benzocyclobutene resins, bismaleimide triazine resins, acrylic resins, and cyclic olefin resins.
By using the material to constitute the organic insulating film which contains at least one resin selected from the group consisting of polyimide resins, epoxy resins, benzocyclobutene resins, bismaleimide triazine resins, acrylic resins and cyclic olefin resins, a thin-film circuit substrate with a high reliability, provided with the organic insulating film having a high bonding strength for metal wiring, can be obtained.
A method of producing the above-described thin-film circuit substrate in accordance with the present invention includes a step of irradiating the surface of the organic insulating film with at least one kind of ions selected from the group consisting of He ions, Ne ions, Ar ions, Kr ions, and Xe ions, such that the surface modification layer having a surface modification coefficient of 0.1 to 0.5 is formed.
By irradiating the surface of the inorganic insulating film with at least one kind of ions selected from the group consisting of He ions, Ne ions, Ar ions, Kr ions, and Xe ions, the surface modification layer having a surface modification coefficient of 0.1 to 0.5 can be securely formed at the surface of the organic insulating film without deterioration of the characteristics which may be caused by excess oxidation at the surface of the organic insulating film.
Moreover, a method of producing the above-described thin-film circuit substrate in accordance with the present invention includes a step of irradiating the surface of the organic insulating film with plasma containing at least one of N2 and Ar such that the surface modification layer having a surface modification coefficient of 0.1 to 0.5 is formed.
By irradiating the surface of the inorganic insulating film with plasma containing at least one of N2 and Ar, the surface modification layer having a surface modification coefficient of 0.1 to 0.5 can be securely formed at the surface of the organic insulating film without deterioration of the characteristics which may be caused by excess oxidation at the surface of the organic insulating film.
A method of producing the above-described thin-film circuit substrate in accordance with the present invention includes a step of irradiating the surface of the organic insulating film with UV rays such that the surface modification layer having a surface modification coefficient of 0.1 to 0.5 is formed at the surface of the organic insulating surface.
By irradiating the surface of the inorganic insulating film with UV rays, the surface modification layer having a surface modification coefficient of 0.1 to 0.5 can be securely formed at the surface of the organic insulating film without deterioration of the characteristics which may be caused by excess oxidation at the surface of the organic insulating film.
The high frequency module in accordance with the present invention uses the above-described thin-film circuit substrate.
By forming the high frequency module for use in millimeter wave or microwave regions by using the above-described thin-film circuit substrate, a high frequency module with a high reliability, having a high bonding strength of the metal wiring to the organic insulating film, can be provided.